1. Field of the Art
The present invention relates to an induction motor control system and more particularly, to an induction motor control system capable of smoothly switching an induction motor driving power supply between a commerical power supply and a variable frequency power supply.
2. Prior Art
A switching system in which, while an alternating-current motor is driven by the power supplied from a power supply with a predetermined frequency (to be referred to as "a commercial power supply" hereinafter in this specification), the output voltage of a variable frequency power supply (to be referred to as "a frequency changer or converter" hereinafter in this specification) is made to coincide with the voltage, frequency and phase of the commercial power supply and then the frequency converter is connected to the alternating-current motor while the commercial power supply is disconnected therefrom so that the alternating-current motor is driven by only the power supplied from the frequency converter, is called a commercial synchronous exchange (to be referred to as "exchange" hereinafter in this specification). Conversely, the systems for switching an alternating-current motor, which has been driven at various speeds by the power supplied from a frequency converter being disconnected therefrom and then connected to a commercial power supply, may be divided into various asynchronous switching systems; a system in which the power supply for the motor is once cut off and then the motor connected to a commercial power supply; a switching system in which an excess current is suppressed through a reactor in the case of switching, and so on, and a switching system in which the motor is disconnected from the commercial power supply or the frequency converter and connected to the frequency converter or the commercial power supply while the commercial power supply and the frequency converter are maintained in voltage synchronization, is called "the commercial synchronous switching system" and is an excellent switching system because when switching, there are no shock torque and no excess current.
However in the case of the synchronous switching of the induction motor, there still exist some technical problems. One problem is that because the induction motor slips, even when the motor is driven by the commercial power supply, the rotational speed of the motor varies depending upon whether a load is heavy or light. Therefore in the case of the induction motors which accomplish the above-described exchange, the V/F control system for controlling the primary frequency and not the rotational speed, has been widely used.
However, the prior art control systems control the output frequency of the frequency converter; that is, the primary frequency of the motor so that it is impossible to control the rotational speed of the induction motor with a high degree of accuracy. Furthermore, the output frequency of the frequency converter is controlled by an open-loop system so that there arises a problem of control system instability in case of acceleration, deceleration or variations in load.